This invention relates, in general to end terminated chip resistors, and more particularly to a high power end terminated chip resistor-printed circuit board assembly.
End terminated chip resistors are small lead-less elements that have been manufactured in a multitude of highly precise values. These resistors have been used extensively in surface mount hybrid circuits where their small package size affords significant savings in printed circuit board space. Moreover, the electrical performance of circuits employing chip resistors is improved since parasitic inductances and capacitances associated with resistor leads are eliminated. In addition, the lead-less feature of these components makes them extremely useful in applications requiring a controlled impedance environment.
Typically, chip resistors have been built by bonding a resistive film to a rectangular shaped ceramic substrate. Electrical contact to the resistive film has been made by a metal coating at each end of the film. The metal coating also covers the ends of the ceramic substrate. The primary purpose of the ceramic substrate is to provide structural support for the resistive film. Collectively, the resistive film, the ceramic substrate, and the metal coating are referred to as the end or wraparound terminated chip resistor.
In most cases, valuable board space is conserved by mounting the end terminated chip resistor directly to bond pads on the printed circuit board. Further, direct mounting has the added benefits of lower cost as well as higher structural reliability and lower parasitic effects owing to the elimination of expensive gold wire interconnects. The primary disadvantage of this method is that the major heat dissipation path is through the surface of the chip resistor to its end terminations. As an example, end terminated chip resistors mounted in this fashion are nominally derated to zero watts at 125.degree. C.; further, a resistor of this type with a length of 2.03 millimeters and a width of 1.27 millimeters is rated at 100 milliwatts at 70.degree. C.
In many applications, thermal dissipation, rather than board space, is the overriding concern in printed circuit board design. To address this issue designers have developed boards with metal lined vias that contact the ceramic substrate of the end terminated chip resistor and have referred to these boards as chip resistor boards in a wire bond configuration. In addition the metal lined vias contact a heat sink. Thus, heat is conducted from the chip resistor to the heat sink through the metal lined via. However, this design requires gold wire interconnects to make electrical contact between the end terminated chip resistor and the signal traces on the printed circuit board. Therefore, not only is board space sacrificed but structural reliability and signal integrity are potentially compromised for improved thermal performance. In this configuration resistors with a length of 2.03 millimeters and a width of 1.27 millimeters are nominally derated to 750 milliwatts at 125.degree. C. Further, at the 70.degree. C. rating there is a seven fold improvement in power rating over the structure in which the end terminated chip resistor is mounted directly to the printed circuit board. Accordingly, it would be beneficial to have an end terminated high power chip resistor-assembly capable of increased power dissipation without compromising board space.